Add instructions on how to run docker image with singularity/apptainer

[runame]

Description

Since many HPCs don't grant root access and have singularity/apptainer pre-installed, it might be helpful to add instructions on how to use the docker image with singularity/apptainer.

[priyakasimbeg]

From email thread; @runame suggests converting Docker images to singularity/apptainer as described in this guide.

[priyakasimbeg]

Fixed in #516